1. Field of the Invention
This invention relates to dual peak detecting circuitry and specifically to optical bar code reading devices using dual peak detection circuitry.
2. Description of the Prior Art
The problem area of the present invention is the use of light pens in reading optical bar code. This problem has been solved in the past by using circuits that interface to the light pens. Light pens traditionally provide a signal representing whether the light pen is passing across a white reflecting surface or a black absorbing surface. Therefore, these interfacing circuits determine when the output is high from the light pen, indicating a white surface or is low from the light pen, indicating a black surface. One such interface circuit is illustrated in the article, "Signal Processing for Optical Bar Code Scanning" by Frederick L. Merkowitz published in Byte, Vol. 1:16, published December 1976, pp. 77-84. Specifically the circuit shown in FIG. 3 provides an output for the bar code reading light pen. Information received by circuitry connected to the light pen is converted into digital information which is used by digital processors elsewhere. Information stored by bar codes traditionally has been price and inventory information.
Light pen interface circuits in the prior art have the capability to digitize the output, i.e., these circuits provide one output state when the light pen is reading a white bar and the other output state when the light pen is reading a black bar. These circuits are called dual peak detector circuits because they detect the high and low peaks of the voltage from the light pens. A dual peak detection circuitry is disclosed in the article entitled, "Two ICs Measure Waveform Levels" by Steven L. Ross, published in EDN, Feb. 17, 1982. This circuit provides the voltage maximum of an input voltage waveform together with the voltage minimum and Vtot which represents the difference between voltage maximum and voltage minimum.
The disadvantage of the circuits in the prior art is that they do not detect with certainty the first transition between a white surface and a black surface. It is the object invention to provide a bar code reading circuit and a dual peak detector circuit that detect with certainty the first transition between the white and dark bar codes.